The broad objective of this proposal are to discover the molecular mechanisms by which chemical changes in nucleic acid pyrimidines, such as photohydrate and photodimer formation, find expression in a change of the biological function or activity of an organism. Specific objectives are: 1. Measurement of the change of binding between guanosine and poly-C caused by chemical alteration of the cytosine rings in the poly-C. In addition we propose further work with the complexes of A and poly-U where the uracil residues have been chemically changed. 2. We propose a continuation in the study of intramolecular energy transfer in our highly successful systems in which a pyrimidine, such as uracil or thymine, is covalently bonded to a detector of the transferred energy, such as a fluorescent group. We propose using molecular elements which will detect transfer of triplet energy, such as alpha-diketone elements, stereospecific alkene groups, and simple ketone elements (at low temperature). BIBLIOGRAPHIC REFERENCES: "A Concentration Dependent Quantum Yield for Photohydration of 1,3-Dimethyluracil," J. G. Burr, W. A. Summers, C. Gilligan, Photochem. Photobiol. 24, 403, (1976). "Emission from the Species CH3 COC(OH)2CH3 in Aqueous Solutions of Biacetyl" Y. J. Lee and J. G. Vurr, Chem. Phy. Letters 43, 146 (1976).